I would probably try a few really good squirts of marvel mystery oil in each cylinder and let it sit for a week. Then after a week, if possible, remove all the plugs, get under the truck, remove the flywheel pan and use a flywheel tool and see if you can rock the motor back and forth. One of the valves was open while it sat and while maybe no direct water in the motor, the humidity over the course of 10 years may have left the rings in that cylinder a little rusty. Hopefully the mystery oil will help get the rust loose and rocking the motor by hand at the flywheel gets the motor free without breaking a ring.
